Luis is 2 times as old as Brandon. 42 years ago, Luis was 9 times as old as Brandon. How old is Luis now?
Explanation: We can use the given information to write down two equations that describe the ages of Luis and Brandon. Let Luis's current age be $l$ and Brandon's current age be $b$ The information in the first sentence can be expressed in the following equation: $l = 2b$ 42 years ago, Luis was $l - 42$ years old, and Brandon was $b - 42$ years old. The information in the second sentence can be expressed in the following equation: $l - 42 = 9(b - 42)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $l$ , it might be easiest to solve our first equation for $b$ and substitute it into our second equation. Solving our first equation for $b$ , we get: $b = l / 2$ . Substituting this into our second equation, we get: $l - 42 = 9($ $(l / 2)$ $- 42)$ which combines the information about $l$ from both of our original equations. Simplifying the right side of this equation, we get: $l - 42 = \dfrac{9}{2} l - 378$ Solving for $l$ , we get: $\dfrac{7}{2} l = 336$ $l = \dfrac{2}{7} \cdot 336 = 96$.